The present invention relates to improvements in solid state power supplies for remote welding operations. In typical applications, a welding power supply will be provided at one location with a long run of cable and a shielding gas line to a remote unit including a wire feeding arrangement and controls for the welder to use at the situs of the weld. A typical application might be a shipyard in which a welder is required to carry the remote welding unit to a plurality of locations for performing arc welding.
Typically, such a power supply is designed for high currents at a relatively low open circuit voltage with typical parameters being a supply for currents on the order of six hundred amperes at open circuit voltages of twenty to forty volts.
Depending on the type of weld being made, the particular shielding gas in use, and various other factors, it is necessary to be able to adjust the open circuit voltage of the welding power supply in use. Previous systems have incorporated variable autotransformers for controlling the output voltage. Such autotransformers dissipated large amounts of heat and were common failure points in such prior art systems.
More recently, solid state AC phase control power supplies using thyristors such as silicon controlled rectifiers (SCRs) and triacs have been used in arc welding applications.
As is known to those skilled in the art, the thyristors used for output voltage and current control in such power supplies are solid state devices having three or more pn junctions wherein a relatively small current at an input terminal called the gate is used to control large currents between the main terminals of the device. The volt-amp characteristics of such devices display a well known sensitivity of the forward breakover voltage required to initiate conduction to gate current.
It is also known that once such a thyristor device is in a conducting state, the internal mechanism which maintains it in its on state is an internal positive feedback provided within the device. When the current through the device (for a particular gate current) drops below a certain level, insufficient current is available to maintain the internal positive feedback and the thyristor will cut off.
It will further be appreciated by those skilled in the art that when an arc is struck by the welder using such a supply, the arc may represent a load on the order of miliohms to the power supply and loss of the arc represent a substantially open circuit. Thus it will be appreciated that the load on the power supply varies greatly in magnitude and quite rapidly when the supply is in use and thus the supply encounters transient variations in current drawn therefrom of large magnitude and short duration.
It is further known to those skilled in the art that in any typical welding application, the environment in which a power supply will be used is subject to extremely high electrical noise levels, many of which are transient in nature and contain large amounts of high frequency energy.
Since relatively small currents into the gates of control thyristors, on the order of less than one amp, are used to control several hundred amperes between the main terminals of such thyristors, and since in its normal environment the output current from welding power supplies varies greatly and rapidly, the main problems encountered in design of solid state thyristor controlled power supplies have included: reliable gate triggering, without spurious triggering due to noise; and maintaining conduction of the thyristor once it is turned on the presence of rapidly varying loads presented to the supply by the arc or termination of the arc.
Prior art solid state welding supplies known to the inventor have used a large number of components in control circuits for controlling the firing pulses to the gates of the main control current thyristors. Typically such circuits will be complex and use a large number of components including integrated circuit operational amplifiers and discrete transistors. In the typical arc welding environment, emitter-base breakdown of transistors has been a common failure point in the control circuits. Furthermore, such previous solid state power supplies have generally had rather complex control circuit for which field repair was difficult. Replacement of high component count circuit boards was often more feasible than repair of the individual boards.
Furthermore, in previous solid state welding power supplies, the problem of maintaining conduction of a thyristor, once triggered in the presence of widely varying loads (and thus widely varying currents through the thyristor) has generally been approached by providing either a plurality of firing pulses in order to maintain conductivity of the thyristor during any one cycle or half cycle of the input voltage, or arrangements for maintaining relatively large amounts of gate current or gate pulses of long duration in order to reduce the holding current of the thyristor to a minimum when conduction is desired.